1. Field of the Invention
This invention relates to the field of electronic video apparatus, and, more specifically, to electronic video test equipment.
2. Background Art
Electronic systems of all kinds require testing for various reasons, such as experimental evaluation, product benchmarking, system certification or verification, and troubleshooting. In the case of video equipment, such testing can be complicated by the one-dimensional transmission mechanism (e.g., streaming analog or digital video) and the two-dimensional display mechanism. Often a test engineer will view a test image on a video display to identify visually apparent degradations in the displayed image. The test engineer may then attempt to trigger an oscilloscope to capture the corresponding portion of the streaming video signal for analysis. Existing mechanisms for triggering the oscilloscope are imprecise and rely on visual guesswork by the test engineer. These problems may be better understood from a general description of video, as provided below.
Video equipment operates on a continuous input stream of data, commonly in the form of distinct color signals or channels (such as R (red), G (green), and B (blue)) or as a single gray-scale signal (equivalent to R, G and B signals having the same values with respect to time) for black and white video. Along with the analog video data, a vertical sync (synchronization) signal and a horizontal sync signal are transmitted to facilitate rasterizing of the stream of video data into a two-dimensional array of values (i.e., “pixels”) that form an image on a display device, such as an analog video monitor. The vertical sync signal indicates when a new image frame should begin (e.g., return to the top-left pixel of the video monitor), and the horizontal sync signal indicates when the display device should begin the next row of pixels.
Like all electrical signals, video signals are subject to the frequency response characteristics of every device or conduit through which the signal is transmitted. One significant effect of the combined system frequency response is that higher frequency components of the signal degrade as the signal passes through cables and video equipment, resulting in distorted display behavior.
For example, a display device may have a scan rate of 40 MHz, which will support image frequencies up to 20 MHz. At 20 MHz, the video signal is swinging between two signal values at each consecutive pixel (i.e., appearing as horizontal stripes that are one pixel wide). If the video signal is passed through a device or conduit that has a roll-off frequency of 17 MHz, for example, signal frequencies near and above 17 MHz will be attenuated. This attenuation causes a reduction in the magnitude of the signal swing at those frequencies that may be visible as a graying or muting of the image intensity in the horizontal stripes described above.
Test engineers that wish to examine this sort of distortion behavior may display an image that contains those high frequency components to look for distortion in the image. Once a distorted location is found, the test engineer may view the corresponding portion of the video signal in an oscilloscope to evaluate the transient response of the system and to determine the level of attenuation where the distortions occur.
However, only a small portion of the video stream is viewable in the oscilloscope display. Therefore, the test engineer attempts to trigger the oscilloscope to capture the video signal as close to the distortion point as possible. In many cases, this means making an educated guess as to the horizontal scan line in which the distortion occurs, and then setting the oscilloscope to trigger on the horizontal sync signal for that scan line. The test engineer must then scroll the display of the oscilloscope to find the location of interest. If the test engineer's scan line guess was inaccurate in the first place, the test engineer will have to make another guess and reset the oscilloscope to trigger on the new horizontal sync signal.
The above method for viewing a desired portion of a video signal on an oscilloscope is time consuming and frustrating for the test engineer. Many engineer man-hours are wasted each year on this awkward testing process, at great expense to the testing company. For this reason, it would be desirable to have a more accurate and efficient method for establishing a scope trigger near a point of distortion in a displayed image.